The Visa Waiver Program (VWP) enables nationals of 36 participating countries to travel to the United States for tourism or business for stays of 90 days or less without obtaining a visa.
Nationals of VWP countries must meet eligibility requirements to travel without a visa on VWP, and therefore, some travelers from VWP countries are not eligible to use the program. Prior to travel, VWP travelers are required to have a valid authorization through the Electronic System for Travel Authorization (ESTA) and must possess a machine-readable or e-passport depending on when the VWP travelers’ passports were issued.
The implementation of the ESTA program allows the Department of Homeland Security to eliminate the requirement that VWP travelers complete an I-94W prior to being admitted to the United States.
Customs and Border Patrol FAQ about the VWP, http://www.cbp.gov/xp/cgov/travel/id_visa/business_pleasure/vwp/faq_vwp.xml